Method for manufacturing semiconductor laser device

ABSTRACT

A semiconductor thin film including a well layer is laminated on a semiconductor substrate, the semiconductor substrate and the semiconductor thin film is cleaved, and a cleavage plane of the semiconductor substrate and the semiconductor thin film, which is obtained by the cleaving, is exposed to an atmosphere produced by decomposition of a gas containing N-atoms under the presence of a heated catalytic substance, thereby a surface layer of the cleavage plane is removed and a nitride layer is formed on the surface. Subsequently, a dielectric film is formed on the cleavage plane.  
     According to the above technique, a natural oxide film formed on the cleavage plane can be removed and also a protective film can be formed by using a catalytic CVD apparatus.

TECHNICAL FIELD

[0001] The present invention relates to a method of manufacturing ahigh-power semiconductor laser device with long-term reliability.

BACKGROUND ART

[0002] A semiconductor laser has been used for apparatuses in variousfields such as information communications, printings, processing,medical applications, or the like. It is necessary to improve power andreliability of the semiconductor laser as a light source, so as toimprove the performance of these apparatuses.

[0003] In general, the semiconductor laser has a structure in which anactive layer is sandwiched between a p-type cladding layer and an n-typecladding layer. Then, a substrate having the layers laminated thereon iscleaved and laser light is generated by applying a current to the activelayer using the cleavage plane as a resonator plane. Then, one of twocleavage planes serving as resonator planes, becomes a light outputtingpart. Further, the two cleavage planes are coated with a dielectric filmfor controlling the reflectance or suppressing deterioration with timecaused by chemical reaction on a cleavage surface.

[0004] When cleaving is carried out in general air atmosphere, a naturaloxide film is formed on the cleavage surface. Taking GaAs compound as anexample, high-density surface levels, which is mainly caused by oxygenbinding of Ga and As, are present in the natural oxide film on thecleavage plane. Then, emitted light is absorbed by the natural oxidefilm as a non-radiative recombination center. Due to the lightabsorption, heat is generated in the vicinity of the cleavage plane, anda forbidden bandwidth of the active region is decreased, resulting infurther increasing light absorption. Consequently, the cleavage plane ismelted away, causing deterioration of the laser output considerably.Therefore, to achieve a high-power semiconductor laser with highreliability, it is necessary to preclude the formation of a naturaloxide film formed on the cleavage plane, particularly.

[0005] Conventionally, to prevent the natural oxide film from beingformed, the following processes are accomplished. That is, aftercleaving is carried out in high vacuum, a protection layer is formedwithout exposing the cleavage plane to the air before forming a naturaloxide film is formed, or after cleaving is carried out in an atmosphericair, the natural oxide film formed on the cleavage plane is removed byan electron beam heating, a laser irradiation, or plasma exposure usingan inert gas so as to form a protection film. In addition, anothermethod is also accomplished. That is, after placing the cleavage planeinto a vacuum apparatus, the cleavage plane is exposed to a halogen gasat 400° C. or higher. Then, an oxide layer is removed by thermochemicalreaction, and a compound semiconductor layer and the like is formedthereon.

[0006] However, the above mentioned cleavage operation in high vacuum isrequired for extremely high vacuum level depending on the process time,resulting in requiring high cost or strict control of apparatuses.

[0007] Further, according to the method of forming a protection film byremoving a natural oxide film by means of an electron beam heating, alaser irradiation, or plasma exposure using an inert gas, the naturaloxide film or surface contaminants is removed by a physical method,mainly. Therefore, there is a concern that defects are introduced in asurface layer in addition to the removal of these. Using the abovemethods, in particular, oxygen binding of Ga and As can be removed,however, the introduced defects function as a recombination center.Consequently, it is necessary to perform precise control of processingconditions or the like for an improvement of these methods.

[0008] Further, according to the method of thermochemical reaction witha halogen gas, since it is necessary to heat the halogen gas to 400° C.or higher, an electrode cannot be formed before the cleavage operation.Instead, an electrode is formed after forming a protection film for theresonator plane which is formed by cleaving. Consequently, there existsa problem in that processes become inconvenient and complicated.

DISCLOSURE OF INVENTION

[0009] The invention is proposed to solve the above problems. Accordingto the invention, a natural oxide film formed on a cleavage plane isremoved and also a protection film is formed by using a catalyticChemical Vapor Deposition (CVD) apparatus.

[0010] Namely, the invention provides a method of manufacturing asemiconductor laser comprising the steps of:

[0011] laminating a semiconductor thin film comprising a well layer on asemiconductor substrate;

[0012] cleaving the semiconductor substrate and the semiconductor thinfilm;

[0013] exposing a cleavage plane of the semiconductor substrate andsemiconductor thin film obtained by cleaving to an atmosphere producedby decomposition of a gas containing N atoms, under the presence ofheated catalytic substances, thereby removing the surface layer of thecleavage plane and forming a nitride layer on the surface; and

[0014] subsequently forming a dielectric film on the cleavage plane.

[0015] According to the invention, even if the resonator plane of thesemiconductor laser is formed by cleaving in the air, a surface layermade of a natural oxide film formed on the cleavage plane is exposed ina vacuum apparatus to a gas containing N-atoms, which are changed intoradical in the catalytic CVD apparatus. By doing this, etching removalcan be carried out at a low substrate temperature with extremely lowlevel of damage of the semiconductor thin film, and at the same time anitride layer, which has excellent chemical stability, can be formed. Asthe gas containing N-atoms, ammonia (NH₃), hydrazine (NH₂NH₂), or thelike can be used. Since the nitride layer has a wide band gap andterminates and decreases defects, it is very preferable material in viewof junction between a semiconductor and a dielectric film. In general,when GaAs is used in III-V group semiconductor laser, however, a GaNlayer is formed therein.

[0016] Subsequently, by forming a dielectric film on the cleavage plane,the dielectric film is formed on the face from which the natural oxidefilm is removed. Because of this, it is possible to prevent temperaturefrom increasing due to light absorption and to prevent the cleavageplane from melting when emitting laser light. Then, since the nitridelayer formed on the cleavage plane, from which the natural oxide film isremoved, has excellent chemical stability, reoxidation will not occureven if the cleavage plane is exposed to the air. Therefore, between thestep of exposing the cleavage plane to the atmosphere produced bydecomposition of a gas containing N-atoms using the catalytic CVDapparatus and the step of forming the dielectric film, it is allowed toexpose the semiconductor substrate to the air.

[0017] Further, in comparison with the case where plasma process such assputtering is used for forming the dielectric film, the method in whichafter eliminating the natural oxide film and forming the nitride film bythe catalytic CVD apparatus then forming the silicon nitride film bymeans of the catalytic CVD apparatus is preferable because plasma damagecaused by ion impacts on the cleavage plane can be eliminated. That is,after removing the natural oxide film and forming the nitride film bymeans of the catalytic CVD apparatus, the silicon nitride film issubsequently formed by using the same catalytic CVD apparatus. Thesilicon nitride film is formed by exposing the cleavage plane to anatmosphere produced by decomposition of a gas containing N and Si, or agas containing N and a gas containing Si, under the presence of heatedcatalytic substances.

[0018] In the invention, it is preferable that a well layer of thesemiconductor laser manufactured by the above steps, is made of acomposition of any elements selected from In, Al, Ga, P and As. Theseelements will form a chemically stable nitride film.

BRIEF DESCRIPTION OF DRAWINGS

[0019] Other and further objects, features, and advantages of theinvention will be more explicit from the following detailed descriptiontaken with reference to the drawings wherein:

[0020]FIG. 1 is a diagram showing relationship between a holder and acleavage plane according to an example;

[0021]FIG. 2 is a schematic view showing a catalytic CVD apparatus andsurroundings thereof utilized in the example;

[0022]FIG. 3 is a schematic view showing a semiconductor laser chipobtained from the example;

[0023]FIG. 4 is an output plot of a semiconductor laser obtained fromthe example;

[0024]FIG. 5 is an output plot of a semiconductor laser obtained fromthe comparative example;

[0025]FIG. 6 shows X-ray photoelectron spectroscopy (XPS) plotsregarding samples such as As3d and Ga3d obtained from the embodiment andthe comparative example;

[0026]FIG. 7 shows an XPS plot regarding a sample such as N1s obtainedfrom the example; and

[0027]FIG. 8 shows XPS plots regarding a sample such as A12p obtainedfrom the example and the comparative example.

BEST MODE FOR CARRYING OUT THE INVENTION

[0028] The invention will be described in detail. A semiconductor lasercomprises a semiconductor substrate, an active region formed thereon, atleast a pair of cladding layers which sandwiches the active region, andp-side and n-side electrodes formed on the upper and lower surfaces, andthe laser is formed on a wafer. Then, the wafer is cleaved into a barshape in the air or nitrogen so as to have a desired resonator length,resulting in forming a semiconductor laser bar. The semiconductor laserbar is put into a vacuum apparatus using a holder so that the cleavageplane serving as a resonator plane is exposed to an atmosphere producedby decomposition of a gas containing N-atoms using a catalytic CVDapparatus. The catalytic CVD apparatus is used for the method ofperforming surface treatment or film formation, in which filament suchas tungsten, which is a catalytic substance, is heated in a vacuumvessel and sprayed with a raw material gas, thereby generating a radicalof the raw material gas by thermal contact decomposition with the use ofcatalytic action. The method is described in more detail in HidekiMatsumura, Jpn. J. Appl. Phys. 37, 3175 (1998), for example.

[0029] First, air is exhausted from the vacuum apparatus in which theholder to be stacked with a semiconductor laser chip is placed, by usinga vacuum pump, to form a vacuum atmosphere at 1×10⁻⁴ Pa or lower.Subsequently, NH₃ gas is introduced therein. In addition, the gas may bediluted with H₂ to control the etching speed of the natural oxide film.The flow rate or pressure of the gas introduced is varied depending onthe pump performance or conditions of the apparatus. In particular, theachieved amount of the radical obtained by decomposition of a gascontaining N atoms varies depending on the distance between the filamentand the substrate and the pressure. Because of this, the substratesurface temperature and the processing time are also varied, so thatoptimization is necessary in respective apparatuses and in respectivecases. For example, when the distance between the filament and thesubstrate is 60 mm, it is preferable that the pressure is approximately0.75 Pa.

[0030] Subsequently, the filament is heated by DC power or the like.When tungsten is used as the filament, the filament surface temperatureis required to be the temperature or higher, at which a gas containingN-atoms can be decomposed. For instance, NH₃ gas will be decomposed at1000° C. While decomposed and generated radical species or decompositionefficiency is varied depending on the filament temperature, heatradiation from heated filament increases the substrate temperature.Since the amount of the temperature increase further depends on thepressure and the distance between the filament and the substrate as wellas the filament temperature, the filament temperature should beoptimized in consideration of the above matters. When the substratetemperature rises, the etching speed is increased. In the case of aradical produced by decomposition of NH₃ gas, the cleavage surface tendsto be coarse. In general, in view of preventing an increase of thesubstrate temperature due to heat radiation, the temperature of 1400° C.or lower is desired as the filament temperature. Further, to prevent anincrease of the substrate temperature, it is effective to coolsurroundings of the substrate by means of water cooling.

[0031] After introducing gas, the filament temperature is increased-tobe the temperature at which a gas containing N atoms can be decomposed.Then, the cleavage plane is exposed to a radical produced bydecomposition of the gas containing N atoms, whereby etching of thecleavage plane can be performed. According to the method, heat contactdecomposition using catalytic action is adopted instead of thedecomposition utilizing high frequency electric field, so that damageswhich accompany ion generation or defects generated on the cleavagesurface due to collision of accelerated ions, extremely become small.Further, nitrogenation of the surface occurs at the same time. Theformation of GaN on a surface layer has an effect of terminating anddecreasing the defects. In addition to this, since GaN has a wide bandgap with respect to an active region composed of a composition of anyelements selected from In, Al, Ga, P, and As, the formation of GaN ispreferable in view of junction between a semiconductor and a dielectricfilm. Furthermore, since GaN has excellent chemical stability, once GaNis formed, reoxidation will not occur even if the cleavage plane isexposed to the air. Therefore, it is possible to transport it in the airat the time of subsequently forming a dielectric film. Consequently,steps become simple. The process time varies depending upon adoptedapparatus as mentioned above, however, it can be optimized by the checkof roughness on the cleavage surface with AFM (intermolecular forcemicroscope) or the binding state of oxygen and nitrogen, which arecomposition elements of the active region, with XPS. As an example, itis preferable to apply the method disclosed in A. Izumi et al./ThinSolid Films 343-344(1999)528-531.

[0032] Further, the portion except for the cleavage plane of thesemiconductor laser bar, namely, upper and lower faces of the bar arecoated with metal electrode while etching process. Etching rate of gold,gold alloy, platinum, or the like, which is commonly used for anelectrode of the semiconductor laser, is extremely slow in comparison tothat of a compound semiconductor. Because of this, when exposing theportion except for the cleavage plane to a radical produced bydecomposition of a gas containing N-atoms, there is no damaged portionon the portion except for the cleavage plane within the range of timerequired for removing an oxide layer on the cleavage surface. Further,when semiconductor laser bars are laminated inside of the holder inorder to expose the cleavage edge face of the semiconductor laserthrough the window portion of the holder, the portion except for thecleavage plane of the semiconductor laser bar is not exposed to theradical produced by decomposition of a gas containing N-atoms. Also, itis possible to prevent adhesion of the film to the portion except forthe cleavage plane in a later film formation step.

[0033] By means of the catalytic CVD apparatus, the oxide layer formedon the cleavage surface is removed by etching using a radical producedby decomposition of a gas containing N-atoms and a nitride layer isformed, and thereafter a dielectric film is formed. Herein, thedielectric film is formed so as to control the reflectance of thecleavage plane, mainly.

[0034] Sputtering, CVD film formation, or the like can be used to form adielectric film. As the dielectric film, an aluminum oxide film, analuminum nitride film, a silicon film, a silicon oxide film, a siliconnitride film, a titanium oxide, or a lamination film thereof ispreferable, and in particular, in order to suppress reoxidation causedby the dielectric film formation process on the cleavage surface, anon-oxide film is more suitable among the above mentioned films for theprotection film which comes in contact with the cleavage plane.

[0035] Then, the cleavage plane is exposed to an atmosphere of a radicalproduced by decomposition of a gas containing N-atoms by using thecatalytic CVD apparatus, whereby a surface layer such as a natural oxidefilm formed on the cleavage plane is removed and at the same time, anitride layer is formed on the cleavage surface. Thereafter, anadditional passivation film maybe formed before forming the dielectricfilm for controlling the reflectance to enhance the passivasion effect.

[0036] When considering the above matter, in comparison with the casewhere the dielectric film is formed by plasma process such assputtering, the following process is preferable because it can preventthe plasma damage due to ion impact onto the cleavage plane at the timeof forming a dielectric film. That is, after exposing to an atmosphereof a radical produced by decomposition of the gas containing N-atoms, asilicon nitride film is subsequently formed by using the same catalyticCVD apparatus. Further, since the silicon nitride film formed by thecatalytic CVD apparatus, has a low film stress in the order of 10⁹dyn/cm², it is preferable in the point that film peeling with time willrarely occur in comparison with the silicon nitride film formed by usualsputtering process. The silicon nitride film can be formed by supplyinga gas containing N-atoms and SiH₄ gas in the catalytic CVD apparatusused for generating a radical produced by decomposition of the gascontaining N-atoms to keep the filament temperature to be not lower thanthe temperature at which filament does not form silicide and not higherthan the temperature at which vapor pressure of filament does not causea problem. For instance, in the case of using tungsten as filament, thetemperature at which film can be formed is within the range of from1600° C. to 1900° C. As the flow rate of the gas containing N-atoms andthe SiH₄ gas, optimum value which makes film stress the lowest value,may be used. Further, when thermal damage, which is caused on thecleavage edge face due to an increase of the filament temperature,becomes a problem, by reducing the film formation time, the siliconnitride film is formed to a thickness serving as a protection layer fromplasma damage, for example, to a thickness of about 2 to 10 nm.Subsequently, a dielectric film having a desired reflectance may beformed by another process such as sputtering.

[0037] The semiconductor laser device according to the invention is notlimited to its epitaxial structure or its composition, and can be widelyapplicable to any structure. To achieve higher power, the semiconductorlaser device may have a structure as cladding layers in which a firstcladding layer and a second cladding layer having a lower refractiveindex and a wider band gap than the first cladding layer, are providedviewed from the active region side, or completely separated confinementstructure in which carrier blocking layers, waveguide layers, andcladding layers are provided on both sides of an active region, andwhich satisfies the relationship that carrier blocking layers have alower refractive index than waveguide layers and cladding layers have alower refractive index than active regions (See U.S. Pat. No.005,764,668A for detail) Further, as the composition of an active regionused for a device, GaAs, AlGaAs, InGaAs, or InGaAsP may be selecteddepending on the oscillation wavelength. Needless to say, anothercomposition may be utilized and in particular, it is preferable to usecomposition having smaller band gap than GaN.

EXAMPLE

[0038] The semiconductor laser has completely separated confinementstructure in which a carrier blocking layer is interposed between anactive region and a waveguide layer, and has a stripe width of 8 μm.Then, the semiconductor laser is designed to oscillates in a single modein the wavelength of 860 nm range comprising a cladding layer made ofAlGaAs, a waveguide layer made of AlGaAs, and an active region formed byhetero junction between AlGaAs and GaAs. A wafer to be formed with thesemiconductor laser is cleaved into a bar-shape in the air so as to forma resonator length of 1.4 mm. Then, some of the semiconductor laser barsobtained by the cleaving, are placed in a holder. FIG. 1 shows the abovestate, more specifically, shows the plane which is exposed to a radicalproduced by decomposition of NH₃ gas by the catalytic CVD apparatus. Inthe holder 1, two semiconductor laser bars 2 a and 2 b and a dummy bar 3are stacked on top of each other in layers so as to expose cleavageplanes of the semiconductor laser bars 2 a and 2 b and the edge face ofthe dummy bar 3, which are formed on the same plane, to the windowprovided in the holder 1. Then, the holder 1 is put into the catalyticCVD apparatus. The catalytic CVD apparatus having the structure as shownin FIG. 2 is used herein. The holder 1 in which the semiconductor laserbars are stacked, is placed on a water-cooled board 5.

[0039] After the vacuum apparatus 12 is evacuated to ultimate vacuum of3×10⁻⁵ Pa by a rotary pump 7 and a turbo molecular pump 6, NH₃ gas of 50sccm is introduced through a flow meter 8 and the pressure of the vacuumapparatus is maintained to be 0.75 Pa by a pressure control bulb 10.Then, the surface temperature of a tungsten filament 4, which ismonitored with an infrared radiation thermometer 9, is heated to 1200°C. with a DC supply 11. By opening a shutter 13, the cleavage plane ofthe semiconductor laser bar, which is exposed to the window of theholder 1, is exposed to a radical produced by decomposition of NH₃ gasfor three minutes. Then, after the treatment for three minutes, theshutter 13 is closed, heating of the filament is stopped, the flow rateof NH₃ gas is increased to be 60 sccm, and subsequently, SiH₄ gas of 1sccm is introduced through a flow meter 14, and filament is again heatedto 1800° C. In the state, the shutter 13 is opened and the cleavageplane of the semiconductor laser bar, which is exposed to the window ofthe holder 1, is exposed to radicals produced by decomposition of NH₃gas and SiH₄ gas for two minutes, thereby the silicon nitride film isformed. At this time, the thickness of the film deposition is about 4nm, which is based on the film deposition speed as conditions studied inadvance. After forming the silicon nitride film, heating of the filamentis stopped, introduction of SiH₄ gas and NH₃ gas is stopped, and thengas is exhausted with a vacuum pump. Subsequently, the holder to bestacked with the semiconductor laser bars is taken out from the vacuumvessel and is turned upside down. Then, the same treatment is performedon the opposite side of the cleavage plane. The holder to be stackedwith semiconductor laser bars, in which both faces of the cleavage planeare treated, is moved to another vacuum apparatus, an anti-reflectivity(AR) coating with reflectance of 2% is applied on both faces of thecleavage plane by sputtering film deposition of an aluminum oxide.Further, high reflectivity (HR) coating with reflectance of 97% isapplied on only one face of the cleavage plane by sputtering filmdeposition of a Si/SiO₂ multilayer film.

[0040] These semiconductor laser bars are cut to form a chip-shape,obtaining a semiconductor laser chip as shown in FIG. 3. On a lightoutputting end surface which outputs a laser light, a silicon nitridefilm 21 and a lamination film 24 made of Al₂O₃ 22 are formed. On theopposite end face, the second lamination film 25 provided with a Si/SiO₂multilayer film 23, is formed. After mounting the semiconductor laserchip on a mounting, to examine the intensity of the end face portion oflight emission, maximum light output is examined by applying CW currentat 25° C. As a result, catastrophic optical damage (COD) level showshigh value of 1.4 W as shown in FIG. 4.

[0041] Further, the surface of the AlGaAs layer of the sample, in whichepitaxial growths of an AlGaAs layer having 2 μm thickness is performedon a GaAs substrate, is treated by a radical produced by decompositionof NH₃ following the above mentioned procedure. Then, the surface isexamined by XPS to check the binding state of surface elements. As aresult, the binding caused by an oxide is not observed on As3d as shownin FIG. 6 and as shown in FIG. 7, N1s peak can be observed on thesample. Further, as shown in FIG. 8, high energy shift can be observedwith respect to Al2p. According to these results, it is confirmed thatelements for oxygen binding are decreased and a nitride layer containingAlGaN as a main component is formed on the surface of AlGaAs.

Comparative Example 1

[0042] The semiconductor laser device, which is the same as that of theexample, is cleaved into a bar-shape in the air and stacked in theholder. Then, the holder is put into the sputtering apparatus, and ARcoating with reflectance of 2% is applied on both faces of the cleavageplane by forming aluminum nitride and subsequently an aluminum oxide bysputtering. Further, HR coating with reflectance of 97% is applied ononly one face of the cleavage plane by forming a Si/SiO₂ multilayer filmby sputtering. After thus formed semiconductor laser bars are cut into achip-shape and mounted on a mounting, maximum light output is examinedin the same manner as the example. As a result, catastrophic opticaldamage (COD) level shows about 1.2 W as shown in FIG. 5.

[0043] Moreover, in the same way as the example, to check binding stateof surface elements regarding the sample in which an AlGaAs layer of 2μm in thickness is formed on a GaAs substrate, the surface of the AlGaAslayer is examined by XPS. As a result, only the binding caused by anoxide is observed on Al, Ga, and As as shown in FIG. 6 and FIG. 8.

[0044] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresent embodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and the rangeof equivalency of the claims are therefore intended to be embracedtherein.

EFFECT OF THE INVENTION

[0045] As described above, according to the invention, a high-powersemiconductor laser device with high reliability can be achieved bytreatment of the light emitting end face using relatively simple method.According to the method, a resonator plane of a semiconductor laser isformed by cleaving in the air and then put into a vacuum apparatus.Then, a natural oxide film formed on the cleavage plane is exposed to aradical gas containing N-atoms produced in the catalytic CVD apparatus,thereby removing by etching and forming a nitride layer at the sametime. Subsequently, a dielectric film is formed on the surface.

1. A method of manufacturing a semiconductor laser comprising:laminating a semiconductor thin film comprising an active layer on asemiconductor substrate; cleaving the semiconductor substrate and thesemiconductor thin film; exposing a cleavage plane of the semiconductorsubstrate and semiconductor thin film obtained by cleaving to anatmosphere produced by decomposition of a gas containing N atoms, underthe presence of heated catalytic substances, thereby removing thesurface layer of the cleavage plane and forming a nitride layer on thesurface; and subsequently forming a dielectric film on the cleavageplane.
 2. The method of manufacturing a semiconductor laser of claim 1,wherein the dielectric film is formed by exposing the cleavage plane toan atmosphere produced by decomposition of a gas containing N and Si, ora gas containing N and a gas containing Si, under the presence of heatedcatalytic substances.
 3. The method of manufacturing a semiconductorlaser of claim 1 or 2, wherein the active layer is made of a compositionof any elements selected from In, Al, Ga, P and As.